1. Field of the Invention
This invention relates to aqueous dispersions for forming heat-curable films and is more particularly concerned with aqueous dispersions comprising amine containing polymers and azetidinedione crosslinking agents, and an improved process for the electrodeposition of said dispersions.
2. Description of the Prior Art
The use of aqueous dispersions for the preparation of coatings, particularly in the field of electrodeposited coatings, is well-known in the art. Cathodic electrodeposition in particular is being widely employed for the coating of such items as automobile bodies and household appliances.
Prominent amongst the coating resins used for cathodic electrodeposition are amine or polyamine containing resins wherein the amine groups are in the form of their acid salts, or quaternary salts and hydroxides. In these cationically charged forms, the resins form solubilized aqueous dispersions usually accompanied by some type of crosslinking agent and when subjected to a direct current in an electrocoating bath they migrate to the cathode which serves as the item to be coated. There the resins and crosslinking agents are uniformly deposited. The result is a heat-curable type of film on the cathode substrate which is thermally converted to the cured coating or film. When the amines are in the form of their acid salts, upon deposition at the cathode they are neutralized and deposited in free base form. Accordingly, when the amines are either primary or secondary they become available in the deposited heat-curable film for various types of curing reactions.
A typical method for cathodically electrocoating solubilized polyamine containing resins is that disclosed in U.S. Pat. No. 3,799,854 wherein the crosslinking agent is a blocked polyisocyanate. At elevated temperatures the polyisocyanate becomes unblocked and reacts with the free amine groups to form ureas.
Another typical method involving the electrodeposition of polyamine resins is that disclosed in U.S. Pat. No. 3,925,181. The crosslinking agent employed is a bis-maleimide wherein the free amines add to the beta-carbon atom of the .alpha.,.beta.-unsaturation of the maleimide thereby effecting a cure of the resin. In a related procedure in U.S. Pat. No. 3,975,251 the crosslinking agent used to cure polyamine resins is one having at least two .alpha.,.beta.-ethylenically unsaturated carbonyl groups (for example, ethylene glycol diacrylate).
In yet another method disclosed in U.S. Pat. No. 3,947,339 wherein blocked polyisocyanates serve as the curing agent, the solubilized amine containing resins also have hydroxyl groups for crosslinking with the isocyanate groups.
We have now discovered coating compositions in aqueous dispersions which employ the prior art primary and secondary amine containing resins in combination with a class of azetidinedione containing crosslinking agents.
The present aqueous dispersions have good storage stability even at elevated temperatures.
Furthermore, the present aqueous dispersions lead to improved methods for electrodepositing the compositions to form heat-curable films on cathodic substrates. No by-products are formed in the curing process and the temperatures required to effect the cure of the deposited films are, generally speaking, below those required previously.